Joey is a horse. He's at war. The end.
Actually, not so fast. Did you notice the contrast in War Horse between the horses and the machines? In one memorable scene, Joey flees in panic as a massive tank barrels down on him; the old ways of war (using horses and close combat) are coming face to face with the new ways (technology and distance).
Crazy Old Friedrich (who isn't that crazy) puts it best: "We should not treat horses like this—we treat our machines better" (14.7). Joey is a horse—a real, living being—and the title reminds us of it.